Subsequent Events - Debt Obligations (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Obligations
Amount borrowed		$ 213,000	$ 201,100	$ 491,000
Payments on debt		154,767	163,667	199,667
Outstanding Balance		667,502	609,391
Available for borrowing		567,167	515,400
Bank Borrowings
Debt Obligations
Amount borrowed		113,000	123,600	291,000
Payments on debt		123,600	137,000	170,500
Outstanding Balance		96,500	107,100
Available for borrowing		503,500	492,900
Bank Borrowings | Subsequent Event
Debt Obligations
Amount borrowed	$ 24,000
Outstanding Balance	120,500
Available for borrowing	479,500
Senior Unsecured Notes
Debt Obligations
Amount borrowed		100,000	77,500	200,000
Payments on debt		31,167	26,667	$ 29,167
Outstanding Balance		571,002	502,291
Available for borrowing		$ 63,667	$ 22,500
Senior Unsecured Notes | Subsequent Event
Debt Obligations
Payments on debt	4,167
Outstanding Balance	566,835
Available for borrowing	$ 67,833